The present invention relates generally to multi-piston engines, such as for pumps or compressors, allowing one to put in motion or to pressurize a hydraulic, gaseous or even a polyphase fluid.
More precisely, the invention relates to a multi-piston engine comprising a drive shaft; a number of cylinders; a number of pistons selectively driven by the alternating motion of the drive shaft and respectively housed in the cylinders in order to delimit, for a fluid put in motion by the engine, a corresponding number of chambers with variable volume, each of which presents a determined maximum volume to the fluid and has a low pressure intake and a high pressure outlet; and a number of delivery channels connecting the high pressure outlets of the respective chambers to the delivery outlet of the engine, each delivery channel having a determined cross section and a determined length, the movement of each piston bringing about a vibrational excitation of the engine, at a determined fundamental frequency, and each chamber, in association with the piston which delimits it and with the delivery channel which connects it to the delivery outlet, forming a corresponding module of the engine.
Piston engines of this type are well known, particularly through French Patent No. 2 655 690, and are very widely used, particularly on motor vehicles for powering assisted hydraulic circuits such as ABS braking systems, automatic transmissions, power steering or suspensions, particularly hydropneumatic suspensions.
Although there are a number of sub-categories of piston engines of this type, which are represented in particular by the radial piston engines and the engines with axial pistons and tilting plates, the design of these piston engines always runs into the problem of reducing the noise and more generally the vibrations produced during operation.
French Patent No. 2 551 505 describes, for example, a pumping system for liquid phase chromatography which aims to reduce the instantaneous flow rate variations and which can possibly obtain a certain reduction in operating noise, at least as an incidental result.
Such a system nevertheless requires both the use of cams with a profile in the form of an arithmetic spiral and of cylinders with great dimensional differences between one another, where such structural constraints are very difficult to accept for engines for industrial use that are mass-produced.
FR 1 546 997 and DE 196 41 779 describe piston engines for industrial use which are primarily designed to operate with a reduced noise level.
The basic principle used in these known engines consists of arranging the cylinders in an irregular manner, so as to introduce an angular offset between the individual points of their respective operating cycles and to attenuate the resulting noise by shifting the phase between the different elementary noises which are emitted.
Nevertheless, this irregular arrangement of the cylinders makes machining of such engines relatively delicate and expensive.
In this context, the invention aims to propose a piston engine which has clearly improved vibrational behavior, without requiring extensive structural modifications of structure with respect to standard piston engines.
To this end, the piston engine of the invention, which otherwise conforms to the generic description stated above, comprises at least a first spectral spreading means associated with a first pair of modules including a first pair of pistons, where this first spectral spreading means is suitable for introducing, between the fundamental frequencies of the vibrational excitations resulting from the respective movements of the pistons of this first pair, a frequency shift equal to no more than 10% of the fundamental frequency of the vibrational excitation resulting from the movement of either piston of this first pair.
Thus, instead of seeking to phase-shift the noise emitted by the ditterent modules, the invention solves the problem of noise reduction due to a relatively modest modification of the frequencies emitted by the different modules.
Preferably, the first spectral spreading means entails a difference between magnitudes respectively associated with the modules of the first pair of modules, each of these magnitudes being represented, for the associated module, by the ratio of the cross section of the delivery channel to the product of the maximum volume of the chamber and the length of the delivery channel.
In order to attain satisfactory effectiveness, the frequency shift introduced between the fundamental frequencies of the vibrational excitations resulting from the respective movements of the pistons of the first pair is equal to at least 1% of the fundamental frequency of the vibrational excitation resulting from the movement of either of said pistons.
Excellent results can be obtained by introducing, between the fundamental frequencies of the vibrational excitations resulting from the respective movements of the pistons of the first pair, a frequency shift of about 2% of the fundamental frequency of the vibrational excitation resulting from the movement of either of said pistons.
The invention thus allows the pistons which respectively delimit the chambers of the modules of the first pair of modules to be identical.
The first spectral spreading means can, for example, entail at least one difference between the maximum volumes of the respective chambers of the modules of the first pair of modules.
For this purpose, it is possible to provide different cylinders of the modules of the first pair of modules, for example, and which are machined in such a way as to have different lengths or diameters, where these differences, however, are then preferably provided outside of the regions in which the pistons move so that the pistons can remain unchanged and identical.
However, the cylinders of the modules of the first pair of modules can also be identical; at least one of these cylinders can then contain a solid filler block which modifies the maximum volume of the chamber defined by this cylinder.
Such a filler block is, for example, formed by a stack of several block elements all having the same volume and produced out of a flexible material such as polyurethane or a compressible elastomer.
The engine of the invention can have as many spectral spreading means as the number of pairs of modules containing neighboring pistons, and even possibly as many spectral spreading means as the number of modules.
In the engine of the invention, the cylinders can thus be arranged in a regular manner with respect to the drive shaft.
The advantages provided by the invention are particularly significant when each delivery channel is separated from the delivery outlet of the piston engine by a non-return valve.
Other characteristics and advantages of the invention will emerge clearly from the description given below, on an indicative and in no way limiting basis, in reference to the appended drawings in which: